TASK-1 channels in oligodendrocytes: A role in ischemia mediated disruption

Abstract

Oligodendrocytes are the myelinating cells of the CNS and, like neurons, are highly sensitive to ischemic damage. However, the mechanisms underlying cytotoxicity in oligodendrocytes during hypoxic/ischemic episodes are not fully understood. TASK-1 is a K+ leak channel that mediates hypoxic depolarisation in neurons. The expression and function of TASK-1 in oligodendrocytes had not previously been addressed. In this study, we investigate the expression of TASK-1 in oligodendrocytes and its role in white matter ischemic damage. Expression of TASK-1 in oligodendrocytes was investigated in the mouse brain using immunostaining. TASK-1 channel function was identified by established pharmacological and electrophysiological strategies, using the whole-cell patch clamp technique in cell cultures of oligodendrocytes from the optic nerve, a typical white matter tract. The role of TASK-1 in hypoxia was examined in isolated intact optic nerves subjected to oxygen glucose deprivation (OGD). Oligodendrocytes are strongly immunopositive for TASK-1 throughout the brain. Patch-clamp identified functional TASK-1-like leak currents in oligodendrocytes using two recognised means of inhibiting TASK-1, decreasing extracellular pH to 6.4 and exposure to the TASK-1 selective inhibitor anandamide. Incubation of optic nerves with methanandamide, a non-hydrolysable form of anandamide, significantly protected oligodendrocytes against hypoxic disruption and death in OGD. Our data demonstrate for the first time that oligodendrocytes express functional TASK-1 channels and provide compelling evidence they contribute to oligodendrocyte damage in hypoxia. Since oligodendrocyte damage is a key factor in ischemic episodes, TASK-1 may provide a potential therapeutic target in stroke and white matter disease.

Expression of Kir4.1 and Kir5.1 inwardly rectifying potassium channels in oligodendrocytes, the myelinating cells of the CNS

“…Astrocytes express carbonic anhydrase (CA) V, while oligodendrocytes have CAII, which is critical for astroglial and oligodendroglial H + regulation by the rapid conversion of CO 2 and H 2 O to H 2 CO 3 , which dissociates to HCO 3 − and H + coupled to plasmalemmal Na + –H + -exchange and Na + -HCO 3 − cotransport, as well as Na + -independent Cl − –HCO 3 − -exchange (Kettenmann et al 1990; Boussouf and Gaillard 2000; Ro and Carson 2004). Kir4.1/Kir5.1 currents are regulated by CO 2 and intracellular pH (Yang et al 2000; Cui et al 2001), hence inhibition of Kir4.1/Kir5.1 channels in response to intracellular acidification during intense axonal electrical activity and high metabolic demand would reduce inward K + currents and help maintain the oligodendroglial membrane potential, which is essential for myelin integrity (Hawkins and Butt 2013). Furthermore, in the retrotrapezoid nucleus of the central chemoreceptors, inhibition of astroglial Kir4.1/Kir5.1 currents is the mechanism by which they sense an increase in extracellular CO 2 and this triggers the release of ATP from astrocytes (Wenker et al 2012).…”

“…Optic nerve explant cultures were prepared from mice aged postnatal day (P)7–11, as described previously (Hawkins and Butt 2013). In brief, optic nerves were carefully dissected and maintained in pre-warmed (37 °C) and pre-gassed (95 % O 2 /5 % CO 2 ) dissecting media, constituted of Dulbecco’s Modified Eagle Medium (DMEM) (Sigma) medium, supplemented with 4 mM l -glutamate, 10 % foetal bovine serum (FBS; Invitrogen) and 0.1 % gentamicin (Invitrogen, Life Technologies Ltd., Paisley).…”

“…For confocal photomicrographs, two-dimensional flattened images of the z -stacks are presented. For colocalization analyses, the technique of Barlow and colleagues was used (Barlow et al 2010), as previously described (Hawkins and Butt 2013), in which the degree of separation between pixels from the red and green channels was determined in single z -sections to provide measurements of signal overlap. First, images were thresholded to separate the positive signal (positive immunolabelling) from background; threshold was determined by measuring the background intensity value for each channel in negative control sections and setting the threshold as the mean background intensity plus three standard deviations (averaged from a minimum of 6 images).…”

“…The results demonstrate that oligodendrocytes, like astrocytes, express at least two subsets of Kir channels, homomeric Kir4.1 channels and heteromeric Kir4.1/5.1 channels. Oligodendroglial K + channels are essential for myelin formation and integrity in the face of depolarizing elements (Hawkins and Butt 2013; Neusch et al 2001; Menichella et al 2006). The special biophysical properties of homomeric Kir4.1 and heteromeric Kir4.1/Kir5.1 channels indicate they have distinct physiological roles in maintaining oligodendrocyte function during the large shifts in K + and pH to which they are exposed during action potential propagation.…”

TASK, TREK & Co.: a mutable potassium channel family for diverse tasks in the brain

“…Immunhistochemical analyses of hippocampus biopsies taken from patients with temporal lobe epilepsy demonstrate increased TASK-1-expression on astrocytes and TASK-3-expression on astrocytes as well as on microglial cells while, on control tissue, both channels can be found predominantly on neurons [43]. In addition, myelin-producing oligodendrocytes express TASK-1-channels which, under hypoxic conditions, contribute to oligodendroglial cell damage [44]. Knowledge on the role of K 2P -channels on non-neuronal cells in the brain is very limited until now.…”